It has long been known that the effective resistivity of certain metals was sometimes substantially eliminated when the metal was exposed to low temperature conditions. Of particular interest were the metals and metal oxides which can conduct electricity under certain low temperature conditions with virtually no resistance. These have become known as superconductors. Certain metals, for example, are known to be superconductive when cooled to about 4.degree. on the Kelvin scale (.degree.K.), and certain niobium alloys are known to be superconductive at about 15.degree. K., some as high as about 23.degree. K.
Discovery of superconductivity in the system La-Ba-Cu-O (J. G. Bednorz and K. A. Muller, Zeit. Phys. B 64, 189-193 [1986]) has stimulated the search for other systems, particularly with a view to substituting other elements for the rare earths (RE) used in the earlier materials. For example, replacement of RE by Bi and Tl has been reported. See M. A. Subramanian et al., Science, 239, p. 1015 (1988); L. Gao et al., Nature, 332, pp. 623-624 (1988).
Michel, C., and Raveau, B., Oxygen intercalation in mixed valence copper oxides related to the perovskites, Rev. de Chim. Minerale, t. 21, pp. 407-425 (1984). This paper discloses and discusses certain oxides of La-(Ca or Sr or Ba)-Cu. It does not mention superconductivity.
European Patent Appln. 87100961.9, filed Jan. 23, 1987, published July 27, 1988 as 0 275 343. Bednorz, Mueller, and Takashige, "New Superconductive Compounds of the K.sub.2 NiF.sub.4 Structural Type, etc." In an example, La-Ba-Cu nitrates are precipitated as oxalates, recovered, and calcined. The invention covers broadly oxide of rare earth-alkaline earth-transition metal. T.sub.c =35.degree. K.
Chu, C.-W., PCT Int. Pub. No. WO88/05029, published 14 July 1988. Int. Appln. No. PCT/US87/02958, filed 9 November 1987. Four U.S. priorities beginning with U.S. Ser. No. 2,089, 12 January 1987. Various oxide combinations are ground together (e.g., ball-milled), pressed into pellets, and sintered to make superconductors. Table I gives T.sub.c 's of 91.degree.-98.degree. K. for eight 3-metal oxides, e.g., YBa.sub.2 Cu.sub.3, LaBa.sub.2 Cu.sub.3, etc.
Ihara, Hirabayashi, and Terada, PCT Int. Pub. No. WO88/05604, Int. Pub. date 28 July 1988; Int. Appln. No. PCT/JP88/00050, Int. filing date 25 January 1988; first of five priorities, Japan, 27 January 1987, 62/17013. A solution of nitrates of, e.g., Ba (or Sr), Y, and Cu is precipitated with oxalic acid. The precipitate is calcined, pressed, sintered, and annealed in oxygen. Alternately dry solid powders (oxide, carbonate, or nitrate) are mixed, pressed, and sintered. T.sub.c as high as 54.degree. K.
Wu et al., Superconductivity at 93.degree. K. in a New Mixed Phase Y-Ba-Cu-O Compound System at Ambient Pressure, Physical Review Letters, 58, 908-910 (2 March 1987) discloses making the title compounds by solid state reaction of Y.sub.2 O.sub.3, BaCO.sub.3, and CuO.
Engler et al., Superconductivity Above Liquid Nitrogen Temperature: Preparation and Properties of a Family of Perovskite-Based Superconductors, J. Am. Chem. Soc. 109, 2848-2849 (1987), mixes Y.sub.2 O.sub.3, BaCO.sub.3, and CuO in a ball mill to give a 1:2:3 ratio of Y, Ba, Cu. The powder was heated in an alumina boat at 950.degree. C., and the resulting black powder was reground and heated again.
Wang et al., Comparison of Carbonate, Citrate, and Oxalate Chemical Routes to the High-T.sub.c Metal Oxide Superconductors La.sub.2-x Sr.sub.x CuO.sub.4, Inorg. Chem. 26, 1474-1476 (1987). This was the only reference we found using a carbonate precipitation technique. The precipitant was K.sub.2 CO.sub.3. According to the paper, it was necessary to wash the precipitate repeatedly, an obvious disadvantage in production work. Washing was necessary because potassium adversely affects superconductivity properties of the finished material. If we wash repeatedly, we remove barium, a highly detrimental loss in our process.
Superconducting oxides of mixed metals including Tl are reported with a T.sub.c in excess of 100.degree. K.
In preparing the system Tl-Ba-CuO, Z. Z. Sheng and A. M. Hermann "Superconductivity in the Rare Earth-Free Tl-Ba-Cu-O System Above Liquid Nitrogen Temperature," Nature, 332, pp. 55-58 (1988), first mixed and ground BaCO.sub.3 and CuO to obtain a product which they heated, then intermittently reground to obtain a uniform black Ba-Cu-Oxide powder, which was then mixed with Tl.sub.2 O.sub.3, ground, and heated, with formation of a superconducting material. It was noted that the Tl oxide partially melted and partially vaporized.
The superconductor system Tl-Ca-Ba-Cu-O was also reported in a paper by Sheng and Hermann, "Bulk Superconductivity at 120.degree. K. in the Tl-Ca-Ba-Cu-O System," Nature, 332, pp. 138-139 (1988). The authors reported "stable and reproducible bulk superconductivity above 120.degree. K. with zero resistance above 100.degree. K." According to the paper the composition was prepared by mixing and grinding together Tl.sub.2 O.sub.3, CaO and BaCu.sub.3 O.sub.4. The ground mixture was pressed into a pellet and heated in flowing oxygen. The result was cooled and found to be superconducting.
See also the paper by Hazen et al., "100.degree. K. Superconducting Phases in the Tl-Ca-Ba-Cu-O System," Phys. Rev. Lett., 60, pp. 1657-1660 (1988), which refers to two superconducting phases, Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10+ and Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.8+, both with onset T.sub.c near 120.degree. K. and zero resistivity at 100.degree. K. Preparation included grinding together Tl.sub.2 O.sub.3, CaO, and BaCu.sub.3 O.sub.4 (or Ba.sub.2 Cu.sub.3 O.sub.5), followed by heating.
And see "Nota Bene" in High T.sub.c Update, Vol. 2, No. 6, p. 1, Mar. 15, 1988, further re properties of the Tl-Ca-Ba-Cu-O system.
Oxides of La-Sr-Co as cathodes for CO.sub.2 lasers are disclosed by N. Karube et al., "Sealed-off CO.sub.2 lasers with La.sub.1-x Sr.sub.x CoO.sub.3 oxide cathodes," Appl. Phys. Lett., 43, 1086 (1983); and N. Iehisa et al., "Performance Characteristics of sealed-off CO.sub.2 laser with La.sub.1-x Sr.sub.x CoO.sub.3 oxide cathode," J. Appl. Phys., 59, 317 (1986). However, the herein-described method of preparing such materials is believed novel.
Salutsky et al., J. Am. Chem. Soc. 72, 3306-3307 (1950); and Quill et al., Anal. Chem. 24, 1453 (1952) disclose precipitation of metal carbonates by heating aqueous solutions of the metal chloroacetates.
Kirk-Othmer, Ency. of Chemical Techn., 4, 843, 2nd Ed. (1964), discusses rare earth sources and describes didymium as a rare earth mixture that excludes Ce and Th.
British nomenclature appears to be somewhat different, in that didymium excludes lanthanum. According to British writer, R. J. Callow, The Rare Earth Industry, p. 46, Pergamon Press (1966), removal of cerium from a rare earth mixture leaves " . . . mainly lanthanum and didymium. In American practice it is called didymium at this stage." (We follow the American definition. See infra.).
From the technical viewpoint it may seem obvious that co-precipitated carbonates would provide enhanced homogeneity. However, the technical solution to the problem proved frustrating and was marked by an initial series of failures. We noted that the Wang et al. process, using potassium carbonate (or sodium carbonate) necessitated numerous washings and apparently left detectable amounts of alkali in the ceramic base even so. As noted, serial washings remove Ba, and would be unworkable in our process. Nor is it merely sufficient that the carbonate be derived from a cation that would burn off completely. For example, ammonium carbonate does not work, because a pH below 7 is required to prevent formation of copper tetraamine, but under these conditions bicarbonate ion is formed, with consequent formation of barium bicarbonate, which, being slightly soluble, disrupts the desired stoichiometry. Quaternary ammonium carbonates, on the other hand, form the desired metal carbonates simply and cleanly without troublesome side-formation of complexes or coordination compounds, with firm and precise retention of the intended stoichiometry; which do not interfere with filtration, or similar recovery operations; which do not introduce contaminants into the final product; and which result in homogeneous particles that do not require grinding. The reaction in the reaction mixture is, of course, ionic; that is, carbonate ions react with the respective metal ions to form the respective metal carbonate precipitate.
A composition having an approximate unit cell formula of YBa.sub.2 Cu.sub.3 O.sub.z, where z is typically about 7, and various related materials, represents a particularly promising group of ceramics for superconducting applications. The compositions are typically formulated from precursors which can be mixed to provide the desired ceramic. In one formulation for these ceramic materials (see, e.g., Wu, Engler, Chu, opera cit) carbonate and/or oxide powders of the solid elements are mixed and raised to a temperature of about 1,000.degree. C., driving off volatile materials, such as carbon dioxide. The mixture is reground and reheated, ordinarily several times to improve the intimacy of the mixture, and then can be pelletized, sintered for several hours, and then gradually cooled to below 250.degree. C. This repeated regrinding tends to introduce impurities, as hereinafter discussed.